In the state of the art, for determining fill level and other process variables of a medium, so called oscillatory forks (e.g. EP 0 444 173 B1), single rods (e.g. WO 2004/094964 A1) and also membrane oscillators are known. Exploited in the respective measurements is the fact that the characterizing variables of the mechanical oscillations (oscillation amplitude, resonance frequency, phase difference over frequency) of the oscillatable unit depend on the contact with the medium and also on the properties of the medium. Thus, for example, frequency or amplitude of the oscillations decreases when especially a liquid medium reaches and at least partially covers the oscillatable unit. The liquid medium acts on the oscillating body of the sensor—i.e. on, for example, the oscillatory fork, or the single rod, or the membrane, as the case may be—and does so, on the one hand, as extra mass moved along with the oscillating body of the sensor, so that the oscillation frequency sinks, and, on the other hand, as a mechanical damper, so that the oscillation amplitude decreases. Therefore, from the decrease in the oscillation frequency or the amplitude, it can be ascertained that the medium has reached a fill level dependent on the physical form and on the position of mounting of the apparatus. Furthermore, the oscillation frequency is dependent also, for example, on the viscosity of the medium (see e.g. EP 1 325 301).
The previously described sensors are frequently used as limit level switches. If the process variable is, for example, fill level, the sensor then produces a signal, which shows that the fill level, which is predetermined by the physical form of the sensor and its location of mounting, was reached or subceeded (fallen beneath).
Such mechanical, or vibronics-based, limit level switches or measuring devices for liquids and bulk goods usually make use of a mechanical resonator (the oscillatable unit) and a drive. The resonator determines the resonance properties of the sensor and reacts to the medium with a frequency change and/or with an amplitude change. The drive produces the mechanical oscillations in the resonator, and serves as a feedback element for the electronics which electrically controls the sensor. Thus, the designations “drive” and “transducer” unit can essentially be used synonymously.
The drives most often make use of piezoelectric elements in the form of stacked disks or rings. Such piezoelements are made, for example, of ceramic lead-zirconate-titanate (LZT). In given cases, ceramic insulating discs are also provided, which galvanically isolate the piezoelements from the housing. For such piezo drivers, a mechanical prestress is required, which serves for the force-based (e.g. frictional) connection of the transducer unit with a mechanical resonator, i.e. with the mechanically oscillatable unit. If the mechanical prestress gives way or is no longer high enough, the sensor can stop working and, for example, no longer produce switching signals. Therefore, in the specification region of the sensor, the mechanical prestress must remain within certain limits.
Typical drives for vibratory limit switches are described, for example, in the following documents: DE 1773 815, DE 3348 119, DE 4118 793, DE 39 31453, DE 1002 3302, DE 42 01360, DE 10 2004 009 495, DE 10 2006 046 251, DE 103 21 025, DE 101 29 556 or EP 1 134 038.
Especially critical is the temperature behavior of a piezo stack drive. The ceramic materials generally have markedly smaller coefficients of thermal expansion than the steel alloys which are used for the housing surrounding the stack drive and, for example, also the feedback electronics. In the case of increased temperatures, a metal sensor housing and the metal securement elements expand to a greater extent than the ceramic in the piezo-stack. This leads to a relaxing of the drive and, as a result, to sensor failure.
The transducer unit is, in such case, prestressed against a membrane in the direction of the medium or the process. In such case, the membrane is, for example, the mechanically oscillatable unit itself, or, for example, the two fork tines of an oscillatory fork are secured to the unit.
In a variant, via the use of the spring properties of the membrane, it is possible to hold the prestress in the drive approximately constant in specific temperature ranges. This functions well, for example, in a temperature range of −40° C. to 150° C. for particular membranes with a diameter greater than 30 mm and a thickness of, for instance, 1 mm.